Happy New Year unreleased version 2.6.0

Hi,

I've just pushed the latest unreleased version v2.6.0 [1], with 
accordingly updated changelog [2]. The main purpose of this unreleased 
version is to mark the latest point where the API 201, compatible with 
v2.2.x of rdiff-backup, has been kept intact.

Hence you're called to test that it's really the case, and validate that 
it works against 2.2.whatever you are currently using. For this you can 
use the current weekly release [3].

 From now on, I'll start to simplify and (hopefully) improve the code 
without caring about compatibility (but still keeping the API under 
control, have no fear!), until we'll come to version v3.0 (and API 300), 
which should make it simpler for me, and hopefully others, to add new 
features.

Under the hood, I've already modernized (Python versions, 
pyproject.toml, etc) and completed our test suite (use of black and 
bandit), progressing our OpenSSF compliance from 81% (IIRC) to 96% [4], 
and paving the way for the evolutions to come.

And, last but not least, I wanted to thank our faithful users and wish 
them all a Happy New Year 2024, independently if you follow or not the 
Gregorian calendar!

KR, Eric

[1] https://github.com/rdiff-backup/rdiff-backup/tree/v2.6.0%2Bunreleased
[2] 
https://github.com/rdiff-backup/rdiff-backup/blob/v2.6.0%2Bunreleased/CHANGELOG.adoc#new-in-v2-6-0-never-released
[3] https://github.com/rdiff-backup/rdiff-backup/releases/tag/weekly
[4] https://www.bestpractices.dev/en/projects/6072